Early in the development of fluid control systems for surgical procedures, the practice was to use sets of flexible tubing lengths that were threaded through different positions in a console to control the flow of irrigation fluid to a surgical site, as well as the flow of aspirated matter from the site. The console included displaceable or fixed clamps and variable pumps to control flow paths and flow rates. In ophthalmic surgery, for example, the irrigation and aspiration lines were coupled to a handpiece, often having an ultrasonically vibrated tip, used by the surgeon for incision and for extraction of matter. Irrigation fluid passed through one or more orifices in the handpiece tip and aspirated fluid was withdrawn through other orifices in the tip.
Originally, the tubing lines would be manually threaded past actuator clamp positions, and the aspiration line would feed to a peristaltic pump or suction source. The next logical design improvement was to arrange the control console so as to incorporate clamps and suction sources in geometries such that they could be accessed by flexible tubing within an enclosure or cassette. In one type of cassette, as shown by Cook U.S. Pat. No. 4,627,833, the cassette housing included back-up surfaces against which flexible tubing could be pinched down to closure through an opening in the side of the cassette by an external actuator, while suction was drawn by an external source through a collection vessel, so as to cause the aspirated fluid to flow into the collection vessel. In another example, evidenced by Steppe et al., U.S. Pat. No. 4,713,051, this approach was modified to provide a cassette with a sidewall which was configured with a series of clamp openings and a separate concave back-up surface for tubing to be engaged by a peristaltic pump. Both of these systems incorporated accessible apertures for fluid vents, although Cook vented air and Steppe vented liquid. The vent apertures were used to cooperate with pressure sensing or pressure relief units in the console, enabling the system or surgeon to relieve pressure and prevent damage to sensitive eye tissues.
These cassettes were primarily intended to be disposable, because blood or blood-containing matter in tubing creates an unacceptable possibility of cross-contamination, unless effective sterilization procedures are used. Adequately safe sterilization procedures often are not regarded as sufficiently cost effective in this country. Nonetheless, under some circumstances and in some countries, sterilization of cassettes and repeated reuse are preferred.
It has also been recognized that the functions of the tubing in a cassette are separable and that only certain parts of the tubing need be disposable or resterilizable. Thus, the relative convenience of single-step insertion of a unitary cassette may provide no real advantage over a unit requiring more than one step, if the additional steps are inherently easy to perform. This benefit is exemplified by Wortrich U.S. Pat. No. 5,163,900, in which the configuration of the receptacle for the cassette is modified by a non-disposable adapter. The adapter is configured so as to allow simplification and substantial cost reduction of the disposable cassette that fits in it. The benefits are also illustrated by Wortrich U.S. Pat. No. 5,282,785, in which the disposable cassette is also substantially reduced in cost and size while the adapter section forms a convenient handling and loading tray. Placement of the aspiration tubing relative to a peristaltic pump is facilitated by a surface guide groove in the adapter.
These approaches demonstrate that by awareness of practical conditions at the operative site and by internal separation of functions, significant cost advantages can be realized for different systems with no meaningful penalty in convenience.
There is yet another type of console using fluid flow control for ophthalmic surgery applications. The system and the related cassette are shown in Beuchat et al. U.S. Pat. No. 5,267,956, the commercial version being that depicted in FIG. 27 in that patent. In this Beuchat et al. system, the cassette is configured with a flat distal face having an outline that fits within a portal in the console. The front face of the mounting portal terminates at a bottom interface slot through which upwardly directed actuator clamps extend, when desired, to close off passageways in a cassette mounted in the portal. A system-coupled sensing aperture is disposed between the two clamps to enable communication between the cassette interior and a vacuum sensor in the console. To allow the pressure level to be communicated from the cassette interior to the console without the danger of fluid return into the cassette, a largely hydrophobic filter is included in the conduit in the cassette. To prevent locking up the control system while limiting reverse flow, the filter includes a small hydrophobic area and is thus known as a "spotted hydrophobic filter". The top of the cassette includes a loop of flexible tubing that seats around the periphery of a peristaltic pump in the upper portion of the portal.
The Beuchat et al. cassette has integrally molded internal passageways and uses an elastomeric insert along its bottom surface that stretches under clamp action to close off passageways. The cassette can be conveniently mounted in place, but has a number of limitations. The intricate internal and integrally molded passageways are difficult to sterilize thoroughly, so that reuse is usually not feasible. The integrally molded passageways are also not capable of accommodating pressure differences by expansion or contraction, so that an interior "capacitance chamber" must be provided to allow for pressure differences. The interior spotted hydrophobic filter, once used, cannot be cleaned of contaminants, so that the cassette itself cannot be reused.
The rigid internal passageways and the complex internal configuration mean that this type of cassette is not suitable for reuse in those countries which widely practice resterilization because of preference or economic necessity. In addition to the complex internal flow passageways, resterilization is precluded by the presence of the hydrophobic filter that is used in the pressure sensing line to the console. It is obviously desirable to utilize the same basic cassette for disposable and resterilizable modes, if these limitations can be overcome.
Another option which is often desired by users of these cassettes is the ability to chose either air venting or fluid venting for suction release of the aspiration line. In ophthalmic systems, means have been provided for many years to counteract the danger of excessive suction being applied through the aspiration line into the eye. A technique developed at an early stage in the use of ophthalmic surgery kits was to provide a coupling line, normally closed by a clamp, between the aspiration line and the source of irrigation fluid. When it was required to relieve the suction pressure quickly, relief of the clamp released irrigation fluid into the aspiration line, breaking the suction and avoiding the danger.
Some users, however, prefer to break the suction by venting to air, which is less instantaneous, because of the compressibility of air and the flexibility of the lines. Surgeons vary in their preferences as to the type of venting to be used, and the preferred mode can vary with the stage of the surgical procedure. Whereas liquid venting forces cortical material away with an abrupt flow, the action of air venting is gentler and at lower acceleration. It is evident that a cassette or system which can provide a capability for use of either one of these venting modes, including the option of switching between them, would provide not only substantial cost savings, but also useful operative advantages. Some might prefer to incorporate a vent selection option in the console rather than a cassette. If this can be done, normal operation and use of the cassette should not be affected.